Parasitism and mutualismSpec 9.4
In short
Parasitism is a relationship in which a parasite lives on or in a host and takes food from it, so the parasite benefits and the host is harmed, as with a tapeworm or a flea. Mutualism is a close relationship between two species in which both benefit, such as nitrogen-fixing bacteria in the root nodules of legumes.
The survival of some organisms depends on a close relationship with another species. Two important types are parasitism and mutualism.
| Parasitism | Mutualism | |
|---|---|---|
| Meaning | One organism (the parasite) lives on or in another organism (the host) and takes food from it | Two organisms of different species live closely together and both benefit |
| Who benefits | The parasite benefits and the host is harmed | Both benefit |
| Examples | A tapeworm in the human gut. A flea on a dog. | Nitrogen-fixing bacteria in the root nodules of legume plants. Cleaner fish that eat parasites off larger fish. |
In mutualism, nitrogen-fixing bacteria in root nodules get sugars from the legume plant, and the plant gets nitrogen compounds from the bacteria. Neither can do as well alone.
Say that the host is harmed in parasitism. Do not just say that the parasite lives on the host.
Quick check
Put in order from smallest to largest: ecosystem, population, organism, community.
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Organism, population, community, ecosystem.
Name two abiotic factors that affect communities.
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Any two of: temperature, light, water, pollutants.
What is meant by competition?
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Organisms needing the same limited resource, such as food, light or water.
What is the difference between parasitism and mutualism?
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In parasitism the parasite benefits and the host is harmed. In mutualism both organisms benefit.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026